Capture of gases emitted from power plants is an area of increasing interest. Power plants based on the combustion of petroleum products generate carbon dioxide as a byproduct of the reaction. Historically this carbon dioxide has been released into the atmosphere after combustion. However, it is becoming increasingly desirable to identify ways to find alternative uses for the carbon dioxide generated during combustion.
Carbon capture and sequestration (CCS) is at the forefront of the energy industry. CCS generally encompasses the field of capturing waste carbon dioxide from large point sources, such as refineries or coal fired power plants, transporting it to a storage site and depositing it where it will not enter the atmosphere, such as an underground geological formation.
Synthesis gas or “syngas” is a byproduct a variety of refinery process. Syngas is a mixture comprising carbon monoxide, carbon dioxide, and hydrogen. It is produced by gasification (or burning/combustion) of a carbon containing fuel to a gaseous product. Production of syngas is ubiquitous to refinery processing via the inevitable use of furnaces, boilers, reformers and the like found in hydrocarbon processing. Even in emerging technologies, such as fuel cells, syngas is produced as a byproduct along with electricity, water, and heat.
The Water Gas Shift (WGS) reaction is an important player in CCS and the proper handling of syngas. WGS describes the reaction of carbon monoxide and water vapor to form carbon dioxide and hydrogen.CO+H2O⇄CO2+H2 As can be see, the WGS reaction provides a source of hydrogen at the expense of carbon monoxide. Hydrogen is a valuable product and can be used in hydroprocessing applications, which generally refers to conversion of heavy petroleum fractions into lighter ones via hydrocracking. It can also be used to produce ammonia.
Hydrogen is most abundantly produced by steam methane reformers (SMR) in petrochemical facilities. Steam reforming describes the reaction of methane with steam to produce hydrogen and carbon monoxide.CH4+H2O→CO+3H2 Here, methane is exposed to steam at very high temperatures to form carbon monoxide and hydrogen. In a second stage, additional hydrogen is produced by exposing the carbon monoxide product to the WGS reaction described above.
Sorption Enhanced Water Gas Shift (SEWGS) describes processes where the WGS reaction is combined with CO2 capture. Syngas enters the SEWGS unit where carbon monoxide is treated with steam to produce carbon dioxide and hydrogen. The carbon dioxide is then adsorbed onto an adsorbent producing a nearly pure hydrogen product. Carbon dioxide can then be desorbed and then deposited via the sites CCS facilities.
Conventional SEWGS methods for capturing carbon dioxide tend to reduce the efficiency of the CCS process, due to the additional steam energy required to capture and/or sequester the carbon dioxide. Specifically, conventional processes utilize a costly, energy intensive steam rinse, which creates a large energy penalty on the plant.
U.S. Pat. No. 6,902,602 describes methods for performing separations by swing adsorption where it is desirable to minimize or avoid interaction between one of the components in a gas stream being separated and a component of the gas stream used for purging the swing adsorption apparatus. Separations of hydrogen and carbon dioxide from syngas stream are noted as an example, where it is desirable to avoid contamination of the hydrogen product stream with any oxygen from the typical oxygen-containing purge stream. The separation methods include use of one or more buffer gas steps during a separation, where a buffer different from any other components is used to prevent contamination between steps of a separation process.
U.S. Published Patent Application No. 2012/0125194 describes an autothermal cycle for CO2 capture. A combustion exhaust gas is contacted with an adsorbent bed to adsorb CO2. The CO2 is then removed by contacting the adsorbent with a gas comprising steam. The resulting output gas containing steam and CO2 is conveyed to a vapor recompression system to recover H2O, CO2, and heat. The recovered H2O and heat are then used to provide steam for the sweep gas. The amount of steam sweep gas required for recovery of CO2 is described as being ˜1 mole of steam per mole of input feed gas. The flue gas input feeds are described as having a CO2 content of 15 mol % or less. Thus, the steam/CO2 molar ratio is described as being at least ˜6 moles of steam per mole of CO2. The process is described as recovering at least 90% of the carbon in the combustion exhaust gas as part of the output gas.
Other potentially relevant publications can include U.S. Patent Application Publication No. 20120318533, European Patent Application No. EP 2220338, an article by Reijers et al., Ind. Eng. Chem. Res., 2009, 48, 6966, and an article by Wright et al., Energy Procedia, 2011, 4, 1457, inter alia.